A resolver is widely used to detect a rotational angle of a rotating apparatus such as a rotor of a motor. When a sine wave excitation signal sin ωt is inputted to the resolver, it outputs two output signals obtained by amplitude-modulating the excitation signal according to a rotational angle θ of a rotating apparatus. The excitation signal means a magnetic energization signal. The two output signals are an alternating current signal sin θ·sin ωt and an alternating current signal cos θ·sin ωt, where t is time.
A digital converter calculates the alternating current signals sin θ·sin ωt, cos θ·sin ωt which are inputted from the resolver, and outputs an output angle φ as digital data. Japanese Patent Application Publication No. 2008-219756 (page 4, FIG. 1) discloses a tracking-type digital converter. In the digital converter, an output angle φ is fed back to an input side, and a control deviation ε=sin(θ−φ) is generated. The output angle φ is controlled so that the control deviation ε may become zero.
The control deviation ε is generated as follows. Values of cos φ and sin φ corresponding to the output angle φ are read from a table. The input signal sin θ·sin ωt and the input signal cos θ·sin ωt are multiplied by the cos φ and sin φ, respectively. Then, one of the two signals obtained by the multiplication are subtracted from the other, and sin (θ−φ)·sin ωt is calculated according to the following expression.
                              (                      sin            ⁢                                                  ⁢                          θ              ·              sin                        ⁢                                                  ⁢            ω            ⁢                                                  ⁢            t                    )                ·        cos            ⁢                          ⁢      ϕ        -                            (                      cos            ⁢                                                  ⁢                          θ              ·              sin                        ⁢                                                  ⁢            ω            ⁢                                                  ⁢            t                    )                ·        sin            ⁢                          ⁢      ϕ        =    ⁢                    (                              sin            ⁢                                                  ⁢                          θ              ·              cos                        ⁢                                                  ⁢            ϕ                    -                      cos            ⁢                                                  ⁢                          θ              ·              sin                        ⁢                                                  ⁢            ϕ                          )            ⁢      sin      ⁢                          ⁢      ω      ⁢                          ⁢      t        =          sin      ⁢                          ⁢                        (                      θ            -            ϕ                    )                ·        sin            ⁢                          ⁢      ω      ⁢                          ⁢      t      
The obtained signal sin(θ−φ)·sin ωt is synchronously detected using a synchronization clock generated from the excitation signal sin ωt. As a result, the excitation signal component is removed from the obtained signal sin(θ−φ)·sin ωt, and the control deviation ε is obtained.
A data sheet of Analog Devices, Inc. having the URL shown below discloses a digital converter which converts two inputted signals sin θ·sin ωt and cos θ·sin ωt from analog to digital and performs the entire signal processing after the conversion digitally.
http://www.analog.com/static/imported-files/jp/data_sheets/AD2S1205_jp.pdf>
The above analog-to-digital conversion may be performed using a successive approximation AD converter of 10 to 12 bits, for example. In general, the successive approximation AD converter has a low conversion speed of approximately 2 to 8 μs. When a position of a rotational object whose rotational angle is to be detected is changed during the analog-to-digital conversion processing, a large error may occur between the actual rotational angle θ of the rotational object and the output angle φ of the digital converter.
Moreover, the successive approximation AD converter includes an analog element such as an analog comparator in the interior so that it causes difficulty in increasing the number of bits in order to achieve a higher precision.
Accordingly, such a digital converter has difficulty in performing signal processing at high speed and with high precision.
Further, in such a digital converter, the control deviation ε=sin(θ−φ) obtained by the synchronous detection can be regarded almost as ε=(θ−φ) when (θ−φ) is small. An angular velocity v can be obtained by integrating the control deviation ε. The output angle φ can be obtained by further integrating the angular velocity v. Then, cos φ and sin φ corresponding to the output angle φ are read from the table, and the obtained cos φ and sin φ are fed back to an input side. Such a feedback loop requires two integrator units. In a case where the feedback loop is composed of the integrator units only, the feedback control system may become unstable.
On the other hand, when a delay time exists between the excitation signal and the alternating current signal from the resolver, the phase of the synchronization detection may be shifted, and the sensitivity of the synchronous detection may be reduced. As a result, an error occurs in calculation of the rotational angle, and the precision of the outputted data of the rotational angle is lowered.
Japanese Patent Application Publication No. 2005-147729 (pages 3 to 6, FIG. 1) discloses a digital converter which improves the problem caused by the above-described delay time. This digital converter calculates a delay time of an alternating current signal outputted from the resolver. Based on the calculated delay time, a calculated rotational angle is corrected.
However, in the above digital converter, the delay time of the alternating current signal is calculated only when the resolver is halted. Therefore, in the case where the delay time is changed due to a change of temperature, for example, while the resolver operates, correction of the rotational angle is difficult in accordance with the change of the delay time.